Thickening Gluten-Free Soups

So my husband and I are in the process of opening a cafe and deli (cafe portion is open, deli will be open in 1-2 weeks). There are quite a few customers who are on gluten free diets, and we have some options (like GF pancakes and crepes) that have been well-received.

I know there are a number of GF options for thickening soups or gravies, but I don't really know what would work best. Obviously, some soups are GF naturally, but I'm thinking of a specific example: I made a really yummy soup of caramelized onions, cheddar and beer. I started with a chicken broth, and did not want to use a ton of cream, so I thickened with a flour/water slurry.

I guess I'd have to find a GF beer, too. But for thickening, should I use a GF flour? Agar? Something else?